In situ interfacial growth of zeolitic imidazolate framework (ZIF-8) nanoparticles induced by a graphene oxide Pickering emulsion

Abstract

A novel method of an in situ interfacial growth of nanoparticles induced by a Pickering emulsion was proposed for the fabrication of hollow composites. With the interfacial growth of ZIF-8 nanoparticles at the n-octanol/water interface of a Pickering emulsion stabilized by graphene oxide (GO), the hollow ZIF-8/GO composite was obtained.

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Pickering emulsions, stabilized by the self-assembly of nanoparticles at a liquid–liquid interface, have been widely used in various fields.^1–7 Many amphiphilic nanoparticles such as silica, proteins, metal oxides, and carbon black have been used as the Pickering emulsion stabilizers.^8–11 When the Pickering emulsion droplets are used as microreactors, they can generally induce solid composites with core@shell structures.¹⁰ Particularly, when the in situ interfacial synthesis takes place at the liquid–liquid interface of a Pickering emulsion, not like the normal bulk interfacial synthesis that can produce some blocky nanoparticle composites, hollow composites can be obtained.¹² However, up to now, only in situ interfacial polymerizations at the Pickering emulsion interface have been reported to successfully produce diverse hollow polymer/nanoparticle hybrids.^13–20

In fact, the large liquid–liquid interface of a Pickering emulsion stabilized by nanoparticles would provide a great space for the in situ interfacial growth of a new type of inorganic nanoparticles or inorganic–organic frameworks such as Metal–Organic Frameworks (MOFs) and Zeolitic Imidazolate Frameworks (ZIFs). Similar with the polymerization, when the new resultant nanoparticles grow up and become dense at the interface, they could assemble together with the stabilizer nanoparticles to produce uniform composites with a hollow structure. By varying the types of the stabilizer nanoparticle or the interfacial grown nanoparticle, we could obtain diverse and task-specific Pickering emulsions, nanocomposites, or even hollow composites. Therefore, an in situ interfacial growth of nanoparticles induced by the Pickering emulsion could be a very convenient and efficient fabrication method. However, to the best of our knowledge, few works has been reported based on this novel strategy.

Recently, graphene oxide (GO), with the hydrophobic basal planes of the carbon networks and the hydrophilic oxygen-containing functional groups, has also been reported as a successful Pickering emulsion stabilizer.^21,22 Because of the remarkable mechanical and physiochemical properties of nanoparticle/GO composites, it shows many potential applications in the growing fields such as gases adsorption of H₂S, CO₂, H₂, proteins separations, selective fluorescent probe for metal ions, and lithium ion batteries.^23–30 Mohsen et al.²¹ systemically investigated the conditions for stabilizing GO Pickering emulsion, and further obtained a Pickering emulsion stabilized by the mixture of Ag/GO composite. Very recently, Thickett et al.³¹ prepared a hybrid hollow capsule through a cross-link polymerization at the interface of GO Pickering emulsion; following an incorporation of surface modified TiO₂ nanoparticles, the hollow TiO₂/GO composite was obtained. So far, for most state-of-the-art synthesis of hollow nanoparticle/GO composites by the Pickering emulsion approach, various nanoparticles should be ready-made, or even be surface-modified^8–11 to render the compatibility with GO, which are usually complicate and elaborate.

Besides the Pickering emulsion stabilizers, the key issue for the success of the in situ interfacial growth strategy is to find the suitable nanoparticles which can grow up at the liquid–liquid interface of the Pickering emulsion droplet. Some excellent works of the interfacial growth of nanoparticles at the bulk liquid–liquid interface or gas–liquid interface^32–34 are great inspirations for us. Among them, zeolitic imidazolate framework (ZIF), constructed by lipophilic imidazolate linkers and hydrophilic metal ions,³⁴ could be especially suitable for the interfacial growth at the oil–water interface. ZIF-8, with a large surface area and a good porosity of 11.6 Å cage connected through small apertures of 3.4 Å, has been well studied.^35–39 Because of the flexible pore apertures and amphiphilic surface, it has shown promising selective adsorption capacity towards less polar molecules. Moreover, it also can be a good stabilizer for the Pickering emulsion.²⁶ Very recently, Sun et al.⁴⁰ fabricated ZIF-8/graphene nanocomposites with remarkable fabricated ZIF-8/graphene nanocomposites with remarkable CO₂ storage capacity. However, due to the normal bulk interface synthesis, the exfoliation and dispersion of graphene sheets was extremely difficult and complicated.

Herein, to verify the reliability of the novel fabrication method through the in situ interfacial growth of nanoparticles induced by the Pickering emulsion, GO sheet was selected as the Pickering emulsion stabilizer, and ZIF-8 nanoparticles were expected to grow up at the interface of GO Pickering emulsion droplet. Two-step Pickering emulsification process were used (Scheme 1), including the initial growth of ZIF-8 nanoseeds induced by GO Pickering emulsion to produce ZIF-8-seed/GO composite, and the further growth of ZIF-8 nanoparticle shell induced by the secondary ZIF-8-seed/GO Pickering emulsion to produce hollow ZIF-8/GO composite. To the best of our knowledge, this is the first attempt to fabricate the hollow composite through the in situ interfacial growth induced by the Pickering emulsion.

Scheme 1 Fabrication of the hollow ZIF-8/GO composite through the interfacial growth approach induced by GO Pickering emulsion.

We recorded each step of this approach by the optical micrographs. As illustrated in Scheme 1, with the precursors of ZIF-8 of methylimidazole (MIM) ligands dissolved in n-octanol phase and Zn²⁺ ions in water phase, the n-octanol/water emulsion system is stabilized by GO sheets. The droplets of the n-octanol/water Pickering emulsion (Fig. 1a) show a smooth interface and a relatively homogeneous size distribution, with a diameter about 50 μm. MIM would diffuse towards GO surface due to its weak water solubility, while Zn²⁺ ions would be adsorbed at GO surface via the electrostatic interaction and the coordination with oxygen-containing functional groups. As a result, the interfacial nucleation of ZIF-8 nanoparticle could easily take place at the n-octanol/water interface to produce a ZIF-8-seed/GO composite. The surface of the GO emulsion droplets gradually changes into unsmooth (Fig. 1b) gives the direct evidence that the interfacial nucleation of ZIF-8 nanoparticle occurred at the n-octanol/water interface. All the characteristic peaks of pristine ZIF-8 crystal can be found on the FTIR spectrum of ZIF-8-seed/GO composite (Fig. S1, ESI†), the peaks at 1680 cm⁻¹ (υ_C=O) and 1022 cm⁻¹ (υ_C–O) from carbonyl, carboxylic and epoxy groups, which confirms the presence of GO in the ZIF-8-seed/GO and ZIF-8/GO composites. The bands at 1559 cm⁻¹ related to –CO–NH, confirming the successful growth of ZIF-8 nanoparticle seeds in the composites. The TEM image (Fig. 2a) further reveals that GO sheets, as the stabilizer of the Pickering emulsion, are well exfoliated and extended even in the process of the interfacial growth of ZIF-8 nanoparticles. The cubic ZIF-8 nanoparticles, with an average size of about 50–100 nm, grow up at the GO surface. It is worth mentioning that the relatively low concentration of the precursors in each phase is vitally important to ensure the interfacial growth of ZIF-8 nanoparticle seeds; otherwise, excessively concentrated MIM ligands will quickly diffuse into Zn²⁺ aqueous solution to produce ZIF-8 crystals in the bulk aqueous phase (Fig. S2, ESI†). For the further growth of ZIF-8 nanoparticles to produce the hollow composite, the concentrations of both precursors of ZIF-8 in either oil or water phase should be increased. However, the addition of precursors into the system disrupted the stability of the emulsion. Nevertheless, considering ZIF-8 nanoparticle has been successfully used as a Pickering emulsion stabilizer; meanwhile, the in situ growth of ZIF-8 nanoparticles at the surface of GO sheets made them closely integrated together as a whole, the ZIF-8-seed/GO composite could also be a good co-stabilizer for the Pickering emulsion. The optical micrograph of the secondary Pickering emulsion (Fig. 1c) demonstrates the reality of the ZIF-8-seed/GO composite stabilizer. After the addition of highly concentrated precursors, the secondary ZIF-8-seed/GO Pickering emulsion shows a similar average droplet size of 50 μm in diameter. The ZIF-8-seed/GO Pickering emulsion system is so stable that the one stored for two months exhibits little deteriorations compared with its fresh counterpart (Fig. S3, ESI†). More importantly, the ZIF-8-seed/GO Pickering emulsion system can be stable even at 60 °C, which is the suitable crystallization temperature for the substantial growth of ZIF-8 crystals. Consequently, with much denser ZIF-8 nanoparticles growing at the interface of the emulsion droplets, the hollow ZIF-8/GO composite (Fig. 1d) is finally obtained. Compared with the ZIF-8-seed/GO composite, the TEM image of the part of the hollow ZIF-8/GO composite (Fig. 2b) shows ZIF-8 nanoparticles become much denser and still closely integrate with GO sheets in the shell of the hollow ZIF-8/GO composite. The optical micrograph of dried hollow ZIF-8/GO composite particles (Fig. 1e) and the laser scanning confocal micrographs (Fig. 2c) reveal the spherical hollow ZIF-8/GO composite with a large amount of ZIF-8 nanoparticles dispersed at the surface of the emulsion droplets. All the XRD patterns of the pristine ZIF-8, the dried ZIF-8-seed/GO composite powder, and the dried hollow ZIF-8/GO composite (Fig. S4, ESI†) show almost the same characteristic peaks of ZIF-8, suggesting the well-developed ZIF-8 nanoparticles within GO sheets. Because of the small amount of GO incorporated in the composite, and the high intensity of ZIF-8 peak at 2θ = 8.7 overlapped with the characteristic peak of GO at 2θ = 8.8, there is no characteristic GO peak observed in the XRD pattern of ZIF-8-seed/GO and ZIF-8/GO composites. Besides, determined by the nitrogen adsorption/desorption isotherms (Fig. S5, ESI†), the surface area of the dried ZIF-8-seed/GO composite powder shows a decrease (585 m² g⁻¹) compared with the pristine ZIF-8 (1189 m² g⁻¹). However, with the further growth of ZIF-8 crystals, the surface area of the hollow ZIF-8/GO composite (967 m² g⁻¹) increases significantly. Due to the good porosity, the Pickering emulsion stabilized by ZIF-8-seed/GO composite, as well as the hollow ZIF-8/GO composite would arouse many interests for the further applications.

Fig. 1 Optical micrographs of (a) GO Pickering emulsion, (b) ZIF-8-seed/GO composite, (c) secondary Pickering emulsion stabilized by the ZIF-8-seed/GO, (d) hollow ZIF-8/GO composite, and (e) dried hollow ZIF-8/GO composite.

Fig. 2 TEM images of (a) the part of the ZIF-8-seed/GO composite and (b) the part of the hollow ZIF-8/GO composite, (c) laser scanning confocal micrograph of the hollow ZIF-8/GO composite.

Conclusions

In summary, we reported for the first time the synthesis strategy of an in situ interfacial growth of nanoparticles induced by the Pickering emulsion. Stabilized by the GO sheet, the nucleation of ZIF-8 nanoparticle seeds took place at the interface of the GO Pickering emulsion droplets, producing the ZIF-8-seed/GO composite. When high concentrated precursors of Zn²⁺ and MIM were added into the system, after the secondary emulsification, the Pickering emulsion stabilized by the ZIF-8-seed/GO composite was prepared. Through the further interfacial growth of ZIF-8 crystals induced by the secondary ZIF-8-seed/GO Pickering emulsion, a uniform dense hollow ZIF-8/GO composite was obtained. The type of the nanoparticle stabilizer or the resultant nanoparticle which can be produced at the oil–water interface can be alternatively changed. Therefore, this strategy would have great inspiration for the fabrication of various task-specific composite Pickering emulsions, nanoparticles composites, as well as the hollow composites.

Acknowledgements

Financial support for this work is provided by the National Basic Research Program of China (2013CB733501), the National Natural Science Foundation of China (no. 91334203, 21376074), the 111 Project of China (no. B08021), and the project of FP7-PEOPLE-2013-IRSES (PIRSES-GA-2013-612230).

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Footnote

† Electronic supplementary information (ESI) available: Including the experimental details. See DOI: 10.1039/c5ra02779a

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